1. Field of the Invention
The present invention relates to an engine exhaust gas control system for performing a lean mixture combustion in an air-fuel ratio lean zone and to an engine exhaust control system having an NOx occluding and reducing catalyst for purifying nitrogen oxides (NOx) in exhaust gas produced at the time of the lean mixture combustion.
2. Description of Related Art
In recent years, a lean air-fuel mixture combustion control is used for burning a fuel on the lean side relative to the stoichiometric air-fuel ratio in order to improve fuel consumption. When such a lean mixture combustion is performed, exhaust gas exhausted from the internal combustion engine includes a large quantity of NOx and an NOx catalyst for purifying NOx is therefore necessary. For example, JP patent No. 2600492 discloses an NOx absorbent (NOx occluding and reducing catalyst) for absorbing NOx when the air-fuel ratio of the exhaust gas is in the lean state and releasing the absorbed NOx when the concentration of oxygen in the exhaust gas is reduced, that is, when the air-fuel ratio is in the rich state.
On the other hand, in a system for absorbing NOx produced at the time of the lean mixture combustion by the NOx catalyst, when the NOx catalyst is saturated with NOx, the NOx purifying ability reaches the limit. Consequently, it is necessary to allow the rich mixture combustion to be temporarily performed in order to recover the purifying ability of the NOx catalyst and to suppress the exhaust of NOx.
However, when the lean mixture combustion is switched to the rich mixture combustion, the air-fuel ratio of the mixture near the Nox catalyst does not immediately change to the rich side. Consequently, it is necessary to set the rich time (rich mixture combustion period) rather long to continue the rich mixture combustion for a time including a time required for a gas condition in an exhaust pipe to shift from the lean state to the rich state. In such a case, when the rich mixture combustion is continued, the fuel injection amount is increased excessively, increasing fuel consumption. At the time of the rich mixture combustion, the engine generating torque is larger than that at the time of the lean mixture combustion. Consequently, when the rich time continues long, fluctuation in engine crankshaft rotation becomes large.
In JP patent No. 2586738, an NOx catalyst is disposed in an exhaust pipe and an NOx oxidant (oxidizing catalyst or a three-way catalyst) is disposed on the upstream side of the NOx catalyst. The catalyst on the upstream side generally carries platinum (Pt)--rhodium (Rh), and palladium (Pd), and ceria (CeO.sub.2) as a co-catalyst and the like on a carrier. The oxygen is therefore stored in the catalyst and the stored oxygen reacts with the rich components (such as HC and CO) in the exhaust gas. Accordingly, the necessary amount of rich components cannot be supplied to the NOx catalyst disposed downstream of the oxidizing catalyst. Therefore, when the lean air-fuel mixture is burned, the oxygen is stored in the form of Ce.sub.2 O.sub.3 and PdO, respectively. When the air-fuel ratio becomes rich, the Ce.sub.2 O.sub.3 and PdO are turned into CeO.sub.2 and Pd to release the stored oxygen. At this moment, the released oxygen reacts with the rich components in the exhaust gas so that the air-fuel ratio on the downstream side of the oxidizing catalyst does not change to the rich side. Consequently, the supply amount of the rich components to the NOx catalyst runs short. Thus, the reduction of the NOx occluded in the NOx catalyst becomes insufficient because of oxidizing catalyst.